That's just some cable braid, which is maybe a few sizes too small for this, as it shows through too clearly. Salvaged it off a fancy old CPU power source that went bust, and I think this piece was originally used on a 1mm thick wire so it's stretched to about as large a size as it can be, still shows how flexible the stuff can be!

It's not very pretty, trust me but it's more of a concept and with better materials like the nylon braid, you could arrive to better results. If you don't have any nylon braid lying around, you can basically salvage an old hoodie draw string, or maybe a thick shoelace, as long as it's a cylindrical piece of lace and not a flat tape style lace.